The present invention generally relates to moving a substrate over a stationary plate, and more particularly relates to a method and apparatus for supporting and controlling a substrate traveling over a curved platen or plate where a thin layer of fluid is entrapped between the substrate and the curved plate, such as in an application for drying liquid coatings on a substrate.
Drying coated substrates, such as webs, typically requires heating the coated substrate to cause liquid to evaporate from the coating. The evaporated liquid is then removed. In typical conventional impingement drying systems for coated substrates, one or two-sided impingement dryer technology is utilized to impinge air to one or both sides of a moving substrate. In such conventional impingement dryer systems, air supports and heats the substrate and can supply heat to both the coated and non-coated sides of the substrate. For a detailed discussion of conventional drying technology see E. Cohen and E. Gutoff, Modern Coating and Drying Technology (VCH publishers Inc., 1992).
In a gap drying system, such as taught in the Huelsman et al. U.S. Pat. No. 5,581,905 and the Huelsman et al. U.S. Pat. No. 5,694,701, which are herein incorporated by reference, a coated substrate, such as a web, typically moves through the gap drying system without contacting solid surfaces. In one gap drying system configuration, heat is supplied to the backside of the moving web to evaporate solvent and a chilled platen is disposed above the moving web to remove the solvent by condensation. In the gap drying system, the web typically is transported through the drying system supported by a fluid, such as air, which avoids scratches on the web.
As is the case for impingement dryer systems, previous systems for conveying a moving web without contacting the web typically employ air jet nozzles which impinge an air jet against the web. Most of the heat is typically transferred to the back side of the web by convection because of the high velocity of air flow from the air jet nozzles. Many impingement dryer systems can also transfer heat to the front side of the web. An impingement dryer system, the air flow is highly non-uniform, which leads to a non-uniform heat transfer coefficient. The heat transfer coefficient is relatively large in the region close to the air jet nozzle which is referred to as the impingement zone. The heat transfer coefficient is relatively low in the region far from the air jet nozzle where the air velocity is significantly smaller and tangential to the surface. The non-uniform heat transfer coefficient can lead to drying defects. In addition, it is difficult to uniformly control the amount of energy supplied to the backside of the web because the air flow is turbulent and complex. The actual effect of operating parameters on the drying rate can usually only be determined after extensive trial and error experimentation.
One method of obtaining a more uniform heat transfer coefficient to the web is to supply energy from a heated platen to the backside of the web by conduction through a fluid layer between the heated platen and the moving web. The amount of energy supplied to the backside of the web is a function of the heated platen temperature and thickness of the fluid layer between the heated platen and the moving web. In this situation, the heat transfer coefficient is inversely proportional to the distance between the heated platen and the moving web. Therefore, in order to obtain large heat transfer coefficients which are comparable to those obtained by air impingement drying systems, the distance between the moving web and the heated platen needs to be very small. In many applications, the web must not touch the heated platen to prevent scratches from occurring in the web. However, in some applications a degree of contact between the web and the heated platen is not detrimental to a product produced from the web coated material and high heat transfer rates are required or desired. In these other types of applications, it is advantageous to have the capability of metering away a sufficient amount of the fluid layer to enable the web to contact the heated platen.
For reasons stated above and for other reasons presented in greater detail in the Description of the Preferred Embodiments section of the present specification, a drying system is desired which forms a thin, uniform, and stable fluid layer between the moving web and the heated platen without forced fluid flow. In addition, there is a need for a drying system which can easily control the fluid layer thickness in order to adjust the heat transfer coefficient and thereby the drying rate required for specific products.
The present invention provides a system and method for moving a substrate having a substrate tension over a curved plate at a substrate speed such that the substrate floats over at least a region of substantially constant clearance (H0) between the substrate and the curved plate. H0 is controlled without adjusting the substrate speed and without adjusting the substrate tension.
In one embodiment, H0 is controlled by removing fluid from between the substrate and the curved plate in the region of substantially constant clearance. In another embodiment, H0 is controlled by injecting fluid in between the substrate and the curved plate in the region of substantially constant clearance.
The substrate moves through at least three regions including an inflow region in which the substrate approaches the curved plate, the region of substantially constant clearance, and an outflow region in which the substrate moves from the curved plate. In one embodiment, H0 is controlled by controlling an adverse pressure gradient on the inflow region. In one form of this embodiment, an adjustable upstream idler holding a portion of the substrate is disposed upstream from the curved plate and is adjustable downward to reduce the length of the inflow region and is adjustable upward to increase the length of the inflow region. In another form of this embodiment, replaceable nose-pieces having varying geometry are used, such that one of the replaceable nose-pieces is disposed on an upstream edge of the curved plate to effectively form the front edge geometry of the curved plate. For example, the replaceable nose-pieces could have different radius of curvature or could have varying lengths. In another form of this embodiment, an adjustable flap is pivotally coupled to an upstream edge of the curved plate, such that an angle of the adjustable flap with respect to the curved plate is adjustable. In another form of this embodiment, an adjustable nose-piece is coupled to an upstream edge of the curved plate to effectively form an adjustable front edge geometry of the curved plate.
The system and method according the present invention can be implemented as a drying system, such as a gap drying system. In such a drying system according to the present invention, the substantially constant clearance H0 between the moving substrate curved heated plate is controllable to more efficiently utilize the drying system. Adjusting H0 also permits the heat transfer coefficient between the heated plate and the moving substrate to be adjusted. Adjusting the heat transfer coefficient enables the same coating line to be used for different products which have different drying requirements. In addition, the drying system according to the present invention can form a thin, uniform, and stable fluid layer between the moving substrate and the heated plate without requiring forced fluid flow.